This invention relates to an electroluminescent device with a polymeric charge (hole) injection layer. More specifically, it relates to such a device in which the hole injection layer is formed from a polymer bearing electron-donating substituents. The invention also relates to a process for generating electromagnetic radiation using such a device.
In recent years, a great deal of research has been conducted into electroluminescent materials, that is to say materials which emit electromagnetic radiation (typically visible light) when an electric current flows through the material. Electroluminescent materials are potentially useful for the construction of image display devices, which could be yew thin and lightweight, and could thus advantageously replace cathode ray tubes, gas plasma displays, liquid crystal displays and other types of image display devices.
Electroluminescent devices comprise, at a minimum, a pair of electrodes with a layer of electroluminescent material sandwiched between the electrodes. Several different types of electroluminescent materials are known; see, generally as to development of such materials, International Patent Application No. PCT/GB90/00584 (Publication No. WO 90/13148). The first type to be developed was inorganic semiconductor materials such as gallium phosphide and zinc sulfide. However, such inorganic electroluminescent materials are not readily usable in large image display devices, and many of them suffer from practical drawbacks, including poor reliability. Accordingly, most recent research has concentrated on organic electroluminescent materials.
Many organic compounds, especially polycyclic arenes such as anthracene, perylene, pyrene and coronene, are electroluminescent. However, electroluminescent devices using these monomeric organic compounds suffer from poor reliability, and also present difficulties in preparing the thin layers of the materials needed for use in practical electroluminescent image display devices, and the electrodes needed for electrical contact with such layers. Techniques such as sublimation of the organic material produce layers which are soft, prone to recrystallization and unable to support high temperature deposition of electrode layers, while techniques such as Langmuir-Blodgett film deposition produce films of poor quality, dilution of the active material and high cost of fabrication. Prior art electroluminescent devices formed from these materials, such as that described in U.S. Pat. No. 3,621,321, typically suffer from high power consumption and low light output.
Attempts have also been made to use solid solutions of monomeric organic electroluminescent materials in non-electroluminescent polymers as the active layer in electroluminescent devices; see, for example, U.S. Pat. No. 4,356,429. However, use of such solid solutions carries a substantial risk of phase separation by crystallization of the electroluminescent material out of the polymer, especially in environments where the electroluminescent device may be subjected to large changes in temperature. In addition, often it is difficult to find a non-electroluminescent polymer which can dissolve a large proportion of the active electroluminescent material to form the necessary solid solution.
Accordingly, research has been carried out on electroluminescent polymers having an electroluminescent group incorporated into the polymer itself However, electroluminescent devices based upon such polymers tend to require high operating voltages and accordingly may have relatively low electroluminescent efficiencies. The operating voltages of such electroluminescent devices can be lowered by reducing the thickness of the electroluminescent polymer layer, but if one attempts to reduce this thickness to about 0.1 .mu.m, defects (so-called "pinholes") appear in the electroluminescent layer and act as shorts between the electrodes, thus destroying the electroluminescent properties of the device. To allow the use of thin electroluminescent layers without the formation of pinholes or other defects, it is known to provide electroluminescent devices with a hole injection layer between the anode and the electroluminescent material and/or an electron injection layer between the cathode and the electroluminescent material. For example, U.S. Pat. No. 4,356,429 to Tang describes an electroluminescent device having a metal porphyrin as a hole injection layer and a tetraphenylbutadiene/polystyrene electroluminescent layer. Similarly, Adachi et al., Jap. J. Appl. Phys., 27(2), L269-L271 (1988) describe an electroluminescent device having an electroluminescent layer formed from a polycyclic hydrocarbon, a hole injection layer formed from an aromatic diamine and a electron injection layer formed from a perylene tetracarboxylic acid derivative.
Ito et al, in Chemical Abstracts 120:311,003f (1994) (Abstract of Japanese Pat. Application Publication No. 05-271,652, published Oct. 19, 1993) describe an electroluminescent device having a hole injection layer formed from a cross-linked polymer containing triphenylamine groups. The device is stated to shown good heat resistance and transparency. However, the cross-linked polymeric hole injection layer described by Ito is inconvenient to prepare. Conventionally, each layer of an electroluminescent device is formed by dissolving the material used to form the layer in a solvent, applying a thin film of the resulting solution on to a substrate by spin coating or a similar technique, and drying the coated substrate to remove the solvent and form the desired thin layer. It is not possible to apply the Ito et al. cross-linked polymer in this manner, since in its cross-linked form the polymer is virtually insoluble in all common solvents. Accordingly, the Ito et al. hole injection layer must be prepared by coating the appropriate monomer from solution, drying to form a layer of the monomer and then polymerizing the monomer in situ to form the cross-linked polymer. Such in situ polymerization is in practice inconvenient since it requires the use of additional equipment often not readily available at locations where electroluminescent devices are prepared, and moving the coated substrate to appropriate apparatus may result in contamination of the coated substrate, with possible damage to the integrity of the thin layers.
The present invention provides an electroluminescent device provided with a hole injection layer which possesses advantages similar to those of the Ito et al. hole injection layer but which can be applied without the disadvantages associated with in situ polymerization.